This application focuses on the potential role of adenosine in inflammation in general, and mast cell function in particular. Adenosine has important anti-inflammatory actions, most likely mediated via A2A receptors inhibiting neutrophil activation and adhesion and T-cell activation. In contrast, adenosine also has proinflammatory effects, particularly in the lung. Mice lacking adenosine deaminase express high levels of tissue adenosine and are characterized by chronic lung inflammation and bronchial hyperreactivity. In humans, inhaled adenosine induces bronchoconstriction in asthmatics but not in normals. Mast cells contribute to this effect, and hypersensitivity to inhaled adenosine correlates with indices of lung inflammation. We hypothesize that A2B and/or A3, in contrast to A2A receptors, are responsible for the pro-inflammatory effects of adenosine. In support of this hypothesis, adenosine inhibits mast cells predominantly expressing A2A receptors, and activates mast cells predominantly expressing either A2B or A3 receptors. Furthermore, we found that A2B receptors activate the human mast cell HMC-1, releasing the Th2 cytokines IL-4 and IL- 13, which in turn induce IgE synthesis by B cells. Of interest, both A2A and A2B receptors are often coexpressed in mast cells and other inflammatory cells, but only one of them predominates functionally. In Specific Aim 1 we will test the hypothesis that inflammatory conditions (hypoxia, Th2 cytokines) modulate the differential expression of adenosine receptors in mast cells, from a predominant A2A to an A2B "proinflammatory" phenotype, as suggested by preliminary results. In Specific Aim 2 we will explore intracellular signaling pathways that may explain differential functions of A2A and A2B receptors, and in Specific Aim 3 we will identify the functional domains of the adenosine A2B receptor responsible for such differential coupling. Finally, in Specific Aim 4, we will explore the relevance of adenosine receptor subtypes in an in vivo model of chronic pulmonary inflammation. [unreadable] [unreadable]